1. Field of the Invention
The present invention relates to a thin film transistor including a semiconductor layer in the form of a polysilicon thin film, and more particularly to a thin film transistor that aims at the structure capable of reducing an off-leakage current through a simple manufacturing process, and a manufacturing process and a laser annealing apparatus for such thin film transistor.
2. Description of Related Art
Thin film transistors (hereinafter referred to as “TFTs”) generally have the structure including a gate electrode, a source electrode, a drain electrode, and a semiconductor layer, which are laminated on a substrate. Of those, a TFT using a polysilicon thin film as the semiconductor layer shows high electron mobility and finds its application in low power displays.
The polysilicon thin film serving as the semiconductor layer is formed by uniformly applying laser annealing to at least a region corresponding to a gate electrode out of an amorphous silicon thin film applied onto the entire substrate surface so as to crystallize the region into polysilicon. In this case, the amorphous silicon thin film in regions corresponding to source and drain electrodes are subjected to the annealing treatment like a channel region between the source and drain electrodes, and turned into a polysilicon thin film. This may cause a problem of increasing electric field intensity between the electrodes and in turn increasing an off-leakage current in the TFT.
To overcome such problem, conventional TFTs adopt a lightly doped drain (LDD) structure in which impurities are doped into portions of the polysilicon thin film between the channel region and a region for the source electrode and between the channel region and a region for the drain electrode so as to lower carrier concentration in the doped portions compared to the channel region (see, for example, JP 2007-335780 A).
To that end, the conventional TFTs require the step of doping impurities into the polysilicon thin film, leading to a complicated manufacturing process and high manufacturing costs.